Technical FeaTure

The weakest signal that a wireless receiver can recover is defined by its sensitivity1

Rx sen dBmBW SNR F_( )

log ( )

=−+ + + 174 10 1

where BW is the bandwidth in Hz, SNR therequired signal to noise ratio and F the systemnoise figure. A low-noise amplifier (LNA), asits name implies, improves the receiver sensi-tivity by reducing the cascade noise figure. Fri-is’s equation shows that the noise figure (F1) ofthe first amplifying stage in the receiver chain(that is the LNA) has a predominant effect,while the noise performance of the subsequentstages (such as F2, F3, etc.) are of lesser impor-tance. This is stated asFFFGFGG

2 ()

where Gn is the gain of the nth stage in the
receive chain.

Cellular base stations (BTS) and microwaverelays have detached low-noise amplifier stageslocated up in the aerial tower in order to miti-gate the NF degradation from pre-LNA cableloss. In the BTS architecture, the LNA stageis preceded by a transmit-receive (Tx-Rx) di-plexer for duplexing a common aerial and aninterference filter for preventing out-of-bandblocking or desensitization. However, boththe duplexer and filter have losses that mustbe minimized as they occur before amplifica-tion.2 Therefore, an LNA with an extra marginin noise performance will relax the duplexer-filter’s loss requirement.

A Brief Survey of Low-NoiSeTechNiqueS

In the ‘70s and ‘80s, low-noise microwave
amplifiers were mostly realized using ceram-ic-packaged devices3, 4 because ceramic has
extremely low loss (dissipation factor, tan δ =
0.001) and the stripline leads width could be
matched to the PCB trace to minimize discontinuities. The shift to plastic surface-mount
packaging (SMP) such as a SOT- 23 or SC- 70,
for cost saving reasons, led to packaging that
significantly degrades the noise performance
because of the epoxy encapsulation’s higher